The high molecular weight polyurethane elastomers have a wide range of commercial utility. The long chain polyurethane elastomers have been prepared by reacting, in general, a polyester or a polyether with one or more organic diisocyanates. In the past, the utilization of these materials has been greatly curtailed due to the difficulties encountered in the storage of the elastomers in a form ready for curing. There are generally two kinds of urethane elastomer compositions, a liquid and a solid. The liquid is mixed with curing agents which extend and/or cross link. It is then poured into a mold and cured. While this is very convenient and has found wide use, there is still a need for a solid elastomer which can be processed on standard rubber processing equipment.
It has been previously considered necessary to utilize a substantial excess of isocyanate to provide adequate curing properties in the polyurethane polymers. The excess isocyanate has been found to react even during storage conditions at room temperatures. It has been found necessary that elastomers formed with an excess of diisocyanate must, in general, be used promptly and with great care to facilitate their providing the proper curing properties to form the desired shaped articles. This has resulted in a marked disadvantage and has prevented, in the past, the substantial use of polyurethane polymers made in this manner in the manufacture of complex molded articles.
Alternative methods have been proposed in order to overcome this automatic curing after the preparation of the polymer. Polyurethane polymers have been manufactured with a deficiency or very slight excess of isocyanate. The substantial excess of isocyanate required for curing is added later, preferably just before the elastomer is utilized in the manufacture of a shaped article. This two step addition procedure forms an elastomer having properties which are not as good as those formed with a one step addition.
Polyurethane elastomers containing non-benzenoid unsaturation have been found suitable for curing by a sulfur curing procedure. However, considerable difficulty has been encountered in effecting a cure of these polyurethane elastomers having non-benzenoid unsaturation since the conventional sulfur curing systems which are used for natural rubbers, SBR and neoprene are not effective, the curing cycle being erratic and the times involved being much longer than is permissible for a commercial curing process.
In the sulfur-vulcanization of polyurethane elastomers containing non-benzenoid unsaturation the use of zinc halides as activating agents has been found to be highly effective. However, the use of halides as activating agents has presented problems in handling since they are extremely hygroscopic, difficult to disperse in the elastomeric material because of their crystalline nature and are highly irritating when in contact with the skin. Some of these problems have been partially overcome by the use of zinc halide co-ordination complexes of 2-mercaptobenzothiazole (MBT) or 2,2'-dithiobisbenzothiazole (MBTS). Zinc halide complexes of MBT or MBTS for use as activators in the vulcanization of unsaturated polyurethanes are disclosed in U.S. Pat. No. 2,868,798 to Kehr. Co-ordination complexes of zinc chloride and cadmium chloride with MBT or MBTS for use in the vulcanization of unsaturated polyurethanes is disclosed in U.S. Pat. No. 2,870,157 to Csendes. U.S. Pat. No. 2,846,416 discloses a process for curing polyether-based polyurethanes containing unsaturated side chains which consist of employing a zinc halide complex as described in the above patents.
The use of the above activators has not proved completely satisfactory in curing polyurethanes. The heretofore known zinc halide comlexes do not provide the desired vulcanizates and, due to the necessity of high concentrations of these activators, objectionable blooming and discolorization have resulted. These defects have serious consequences when the vulcanized polyurethane is used in white goods applications, such as in cut thread. Moreover, the prior art complexes have a low weight ratio of zinc halide since the concentration is fixed by the 1:1 complex composition. Furthermore, the complexes are tedious, difficult and expensive to form. Accordingly, there is a definite need for an activator which overcomes the objectionable features described above and which will produce the desired cure rate in a polyurethane elastomer. It is therefore the principal object of the present invention to provide such an improved activator system.